Hydrogeochemical Characteristics and Genesis of Geothermal Water from the Ganzi Geothermal Field, Eastern Tibetan Plateau

Vonsée B., Crijns-Graus W., Liu W.

The majority of energy used by the European Union has been imported from non-EU countries. The EU desires to increase its own renewable energy use to secure its future energy supply. In this paper, an assessment framework of technology dependence has been proposed that can be used to locate bottlenecks in the value chain of geothermal power generation. The framework consists of an ‘above ground’ and ‘underground’ domain. It was applied to binary Organic Rankine Cycle (ORC) power plants as this type has the highest proliferation potential in the EU. The above ground domain aims to locate potential bottlenecks at a key-component level via a technology hierarchy analysis, company and trade analysis as well as a survey. The underground domain focuses on the potential bottlenecks embedded in the geothermal drilling industry by means of a drilling industry screening, rig screening and a survey. The results suggest that some Binary-ORC key-components may require attention. Furthermore, the geothermal drilling industry's dependence on the oil and gas industry can be seen as a major dependence bottleneck that might jeopardize the future proliferation of binary-ORC technologies in the EU.

Li J., Sagoe G., Yang G., Liu D., Li Y.

The Batang geothermal field, located at the north-eastern margin of the Qinghai-Tibet Plateau, is a high-temperature geothermal system with two main geothermal areas, namely Bogexi and Rekeng. The Rehai geothermal field is also a high-temperature geothermal system, with a magmatic heat source which shows geothermal manifestations similar to those in Batang, especially in the Rekeng area. In this study, we compared the chemical compositions of different thermal springs in the Batang geothermal field with those in the Rehai geothermal field, to ascertain whether the thermal spring waters in Batang are also influenced by magma degassing or not. Further, we investigated the hydrogeochemical processes involved in forming the various thermal springs in the Batang geothermal field. We find that the thermal waters in the Batang geothermal field are not influenced by magma degassing. Although inorganic carbon, including HCO3− and CO32−, is the dominant anion component of most of the thermal waters in the Batang geothermal field, SO42− is present in variable amounts. In this area, the mixing of the thermal waters with steam containing some amount of H2S generated by metamorphism probably causes the relatively high SO42− concentration in the neutral thermal springs. Also, beneath the Rekeng area, there is a deep reservoir with the temperature of about 180 °C; a shallow reservoir of a relatively low temperature of about 75 °C was also identified in the eastern part. About half of the thermal spring waters in Rekeng directly originate in the deep reservoir, mixing with little or no snow-melt water and steam. Conversely, no steady reservoir exists beneath the Bogexi area. We attribute the high levels of Ca2+, Mg2+ and TIC (total inorganic carbon) in the thermal spring waters in the Bogexi area to the carbonate geologic stratum distributed at the outlets of the springs.

Li J., Yang G., Sagoe G., Li Y.

The Kangding geothermal field, located in western China, is a high-temperature geothermal system potentially rich in geothermal resources. In this study, we employed various hydrogeochemical methods to gain more insight into the heat source and cooling processes involved in forming various thermal springs in the geothermal field. The study showed that though the majority of the samples analyzed were immature in terms of mineral-aqueous equilibria, coupling classical geothermometers with the FixAl method enabled more reliable reservoir temperature estimations. A deduction from the silica-carbonate, the chloride-enthalpy, and the silica-enthalpy mixing models indicated that the parent geothermal fluid exists beneath this study area. The thermal waters discharged from the Kangding geothermal field originate in the same deep reservoir; and the parent geothermal fluid has a temperature of about 260 °C, with a Cl− concentration of 1056 mg/L. Isotope (δD and δ18O) studies confirmed the magmatic heat effect on the parent geothermal fluid. Also, though all the thermal spring waters in the field are derived from the parent fluid, they undergo different cooling processes during ascent to the earth's surface. However, the thermal spring waters in both the Yulingong and Erdaoqiao geothermal sites were mainly formed by the mixing of the parent geothermal fluid and infiltrating groundwater. The thermal spring waters in the Erdaoqiao also had the highest levels of Ca2+, Mg2+ and TIC (total inorganic carbon) due to the presence of carbonate in its geologic stratum. The thermal springs at Zhonggu formed as a result of the “CO2 condensate”, consisting of snow-melt water and meteoric water, mixing with the deep parent geothermal fluid. We attribute the absence of acid springs in the Kangding geothermal setting to the deep-seated magma chamber and a relatively small concentration of H2S in the deep thermal waters in the area.

Qu W., Lu Z., Zhang Q., Hao M., Wang Q., Qu F., Zhu W.

The southeastern Tibetan Plateau and surrounding areas comprise a typical tectonic belt in mainland China, characterized by a complex geological background and intense tectonic activity. We study present-day crustal deformation characteristics of this region based on GPS data for the periods 1999–2007 and 2009–2011. We first analyze the variations in crustal motion and strain rate, and then discuss the 3D crustal motion and local sub-block activities, as well as the correlation between the intensity of crustal activity and the strain rate distribution. Finally, we explain the present-day geodynamic characteristics of the region. Our results indicate that the entire region shows overall clockwise motion with respect to the stable Eurasian Plate. The tectonic boundary belts between this region and the South China Block display significant compressional strain, accompanied by associated extensional strain. Relatively high maximum shear strain and the transition zones of the significant plane strain gradients are also mainly concentrated along the Ganzi–Yushu–Xianshuihe, Anninghe–Zemuhe–Xiaojiang, Lijiang–Xiaojinhe, and Red River faults, as well as the western and southern Yunnan Province. 3D crustal velocities further reflect significant differences in tectonic activity between different structural belts. We conclude that the regions showing higher shear strain and the transition zones of the significant plane strain gradients correspond to the areas with frequent earthquakes. According to the crustal deformation and strain characteristics, we infer that the present-day geodynamic setting of the region is related to the ongoing India–Eurasia collision and the associated resistance of the stable Alashan, Ordos, and South China blocks, resulting in the extrusion of the southeastern Tibetan Plateau crustal material with an overall clockwise flow around the Eastern Himalayas and significant compressional strain along the tectonic boundary belts. Furthermore, notable compressional strain and enhanced sub-block motions occurred around the Longmenshan fault area following the Wenchuan earthquake.

Shvartsev S.L., Sun Z., Borzenko S.V., Gao B., Tokarenko O.G., Zippa E.V.

The chemical and isotopic compositions and the origin and formation conditions of the nitric and carbon dioxide thermal waters in Jiangxi Province (China) are examined. The differences between these nitric and carbon dioxide thermal waters are shown. The nitric thermal waters are ultra-fresh and high alkaline with abundant SiO2, F, Na, Li, B, Sr, Rb, etc. but low concentrations of Ca, Mg, Cl, Ag, V, Pb, Zn, Co, etc. The carbon dioxide thermal waters are distinguished by higher salinity but lower pH values. The predominant anions are HCO3− and Na+. The thermal waters' composition peculiarity is also determined by SO42−, F−, CO2 and H2S. The special focus is on the thermal waters' origin and the geological conditions of the recharge and discharge zones. The saturation degree of thermal waters with various secondary minerals (carbonates, fluorides, clays minerals, zeolites, pyrogenetic minerals, etc.) is also calculated. The thermal water – rock system is shown to be an equilibrium-nonequilibrium system. While ascent to the surface, studied thermal waters continuously dissolve minerals that are far from equilibrium and form new minerals that are in equilibrium with water. Over time, the solution composition, type of secondary minerals, and chemical element proportions change because some elements precipitate from the solution and the rest continue to accumulate. In nitric thermal waters, the dynamic equilibrium of elements entering and precipitating from the solution is achieved during early stages when the water is ultra-fresh, which creates high pH values and low PCO2. This equilibrium state decreases the total dissolved solids (TDS) growth of nitric thermal waters, which stay low mineralized. Carbon dioxide thermal waters have higher PCO2 and, accordingly, lower pH values, thus achieving dynamic equilibrium during later stages when their TDS exceeds 3 g/l. Therefore, carbon dioxide thermal waters are more mineralized. The origin of redundant elements, particularly F, in thermal waters is considered in the paper, and we show that the source of fluorine is simple minerals of igneous origin.

Tian J., Pang Z., Guo Q., Wang Y., Li J., Huang T., Kong Y.

Rekeng geothermal system in Eastern Himalayan syntax is found to exhibit the strongest surface manifestations in the western Sichuan plateau, with numerous boiling springs, fumeroles and geysers. What is the heat source of such a high temperature system? Is there a magmatic heat source to support it? In this study we have attempted to seek clues from the isotope geochemistry of geothermal fluids. The stable isotope δ2H and δ18O composition of geothermal water suggests that it is recharged by precipitation and snow melt of the surrounding mountains. The chemical type of the geothermal water is alkaline HCO3-Na as a result of water-CO2-rock interaction. Geothermal reservoir temperature in the fractured metamorphic rock is estimated to be between 200 °C–225 °C, using the chemical geothermometers and the chemical thermodynamic modeling approach. During the degassing process upon rising, 0.05 mol/L CO2 has escaped from the geothermal fluid. Evidence from the relationships among major ions and geothermal suite (Li, B, F, As) indicate that the hot springs shared the same parent source fluid and they mixed with cold groundwater to different levels in the subsidiary fractures near surface. Carbon isotope signatures show that the CO2 enriched geothermal gas is 95% of crustal metamorphic origin. Additionally, based on helium isotope analysis, the mantle magmatic 3He signatures have been largely obliterated since it accounts for no more than 5%, implying there is no underlying mantle-derived magma chamber acting as heat source. Therefore, a significant portion of heat is likely converted from crustal deformation in view of the regional tectonic background as Eastern Himalayan syntax.

Shestakova A., Guseva N., Kopylova Y., Khvaschevskaya A., Polya D., Tokarev I.

Zhang X., Hu Q.

Geothermal resources in China are distributed throughout the country, with hydrothermal systems of high temperature in the Tibet Autonomous Region, Yunnan Province and Taiwan Island and hydrothermal systems of low-medium temperature mainly in various sedimentary basins. Development and exploration of geothermal energy in China are below expectations. The purpose of this study is to comparatively review the characteristics (geology, hydrogeology, hydrochemistry and geophysical data) of typical hydrothermal fields/areas and suggest development and utilization approaches in the future. Hydrothermal systems formed by mountain lifting contain a considerable amount of energy for geothermal power generation, especially in the Tibet Autonomous Region, Yunnan Province and Taiwan Island. However, geothermal water in the Tatun geothermal field has high TDS (total dissolved solids), an issue that requires more research to resolve this problem for power generation. The large storage of geothermal resources has been investigated in Meso–Cenozoic sedimentary basins; it is basically used for heating, bathing or greenhouse plantation. Moreover, hydrothermal resources of low-medium temperature can also be used in binary power plants. Although the enhanced geothermal systems (EGS) in China are promising, the resources have not yet been commercially exploited, because the emerging technologies (hydraulic fracturing) and concerns over environmental impacts (induced micro-seismicity) lead to slow development. On the contrary, shallow geothermal energy has been directly utilized mainly for heating and cooling buildings. Cities like Beijing, Tianjin and Shenyang have established a series of ground-source heat-pump systems, which has led to a massive reduction of CO2 emission of 19.87×106 t.

Zhang Y., Xu M., Li X., Qi J., Zhang Q., Guo J., Yu L., Zhao R.

Wang X., Wang G., Lu C., Gan H., Liu Z.

This study defines reasonable reservoir temperatures and cooling processes for geothermal fluids in three representative high temperature geothermal fields – Yangyi, Yangbajing, and Gulu – distributed along the active geothermal belt of Nimu–Nagchu in south-central Tibet. It uses a combined analysis of hydrochemical compositions, chemical geothermometers, and multicomponent chemical equilibrium analyses of geothermal fluid with an enthalpy vs. chloride plot. There are two geothermal reservoirs in Yangyi and Yangbajing, and both the intermediate and high temperature reservoirs of any one geothermal field are essentially within the same hydrothermal system. The subsurface geothermal fluids from Yangyi cooled mainly by mixing with abundant cold water in the intermediate (163–172 °C) and high temperature (192–200 °C) reservoir. The subsurface geothermal fluids from Yangbajing experienced adiabatic cooling and mixing with colder water, which formed the high temperature reservoir ZK4001 (255 °C) and intermediate temperature reservoirs (164–177 °C), respectively, and then emerged on the surface with adiabatic cooling during the ascent. The subsurface geothermal fluids from Gulu ascended to high temperature geothermal reservoirs (211–234 °C) mainly cooled by adiabatic boiling or mixing with cooler water. Most of the high temperature fluids mixed with colder water (mixing temperatures range from 149 °C to 176 °C) during the ascent, and then emerged on the surface as hot springs mainly cooled by conduction. The deep parent fluid of Yangbajing is calculated to have a Cl − concentration of 767 mg L −1 and enthalpy of 1350 J g −1 (water temperature of 321 °C), which agrees well with the maximum temperature measured in well ZK4002 (329 °C). The deep parent fluid of Gulu is calculated to have a Cl − concentration of 845 mg L −1 and enthalpy of 1290 J g −1 (water temperature of 307 °C).

Guo Q., Pang Z., Wang Y., Tian J.

High-temperature geothermal systems hold an enormous capacity for generating geothermal energy. The Kangding area is a typical high-temperature geothermal field in the Himalayan Geothermal Belt. Hydrogeochemical, gas geochemical and isotopic investigations were performed to identify and qualify the main hydrogeochemical processes affecting thermal water composition, including mixing and degassing, and then to estimate a reliable reservoir temperature. Nine water samples and four geothermal gas samples were collected and analysed for chemical and isotopic components. The results demonstrate the alkaline deep geothermal water is the mixtures of approximately 75% snow-melt water and 25% magmatic water. It is enriched in Na, K, F, Li and other trace elements, indicating the granite reservoir nature. The shallow geothermal water is the mixtures of approximately 30% upward flow of deep geothermal water and 70% meteoric cold water. High concentrations of Ca, Mg and HCO3 indicate the limestone reservoir nature. There is no remarkable oxygen isotope shift in the geothermal water since the rapid circulation is difficult to trigger off strong water-rock interaction. CO2 is the predominant geothermal gas, accounting for more than 97% of total gases in volume percentage. The concentration of CO2 degassing ranged from 0.4 mol L−1 to 0.8 mol L−1 via geothermometrical modelling. As a result, the geothermal water pH increased from 6.0 to 9.0, and approximately 36% of the total SiO2 re-precipitate. The sources of CO2 are the metamorphism of limestone and magmatic degassing based on the composition of carbon isotope. The appropriate geothermometers of Na-K and Na-Li yield reservoir temperature of 280 °C. The geothermometrical modelling, developed to eliminate the effects of CO2 degassing, yields temperature of 250 °C. The silica-enthalpy mixing model yields temperature of 270 °C with no steam separation before mixing.

Fusari A., Carroll M.R., Ferraro S., Giovannetti R., Giudetti G., Invernizzi C., Mussi M., Pennisi M.

The geochemistry of thermal spring waters in the Acquasanta Terme area, located on the Adriatic side of central Italy, has been investigated in order to characterize the geothermal feeding system. The springs discharge more than 100 L/s at temperature ranging between 20 and 30 °C. They occur in a tectonic window of Mesozoic limestones in the central sector of the Acquasanta anticline within the Laga foredeep Basin. Chemical and isotopic compositions of thermal and cold fluids were investigated, most of them monitored for one year, in order to understand the thermal fluid circulation paths. The chemistry of the major elements defines the existence of Na–Cl and Ca–Cl–SO42- hot discharging waters and permits the characterization of the thermal end-member hosted in carbonate-dominated reservoir (Burano Anhydrites Fm–Calcare Massiccio Fm). This deep fluid is well represented by the sample T1 and shows high temperature and electrical conductivity (EC), very stable over time, and not affected by mixing phenomena. This is also confirmed by tritium results (0 T.U.). Close to the surface at different depth, such water undergoes mixing or dilution processes with HCO3− rich freshwaters, driven by the complex structural setting of the area and by diffuse karst caves. This is identified as the main reason for observed compositional variations of sampled springs, and three areas affected by different mixing phenomena have been defined at the surface. The concentrations of SO42− and H2S suggest redox processes affecting sulfur after interaction with evaporitic formations, identified with the Burano Anhydrites, at the base of the Umbria–Marche sedimentary sequence (∼3500-m-deep). Contribution from this reservoir is also supported by characteristic Sr isotope signature. δ18O and δD values indicate a meteoric origin of the thermal waters and allow estimation of an average infiltration altitude ranging between 1500 and 1700 m a.s.l. This datum, supported by structural data, suggests the Laga Mountains as the recharge area of the system. Reservoir temperature inferred by SO42−–HCO3−–F, Ca/Mg and SO42−/F geothermometers is about 80 °C, consistent with the geothermal gradient of the foredeep basin, and deserving further investigations for potential economic implications about this low-enthalpy geothermal area.

Chen L., Ma T., Du Y., Xiao C., Chen X., Liu C., Wang Y.

Geothermal energy is abundant in Guangdong Province of China, however, majority of it is still unexploited. To take full advantage of this energy, it is essential to know the information of geothermal system. Here, physical parameters such as pH and temperature, major ion (Na+, Ca2 +, Mg2 +, Cl−, SO42 − and HCO3−), trace elements (Br−, Sr2 +, Li+ and B3 +) and stable isotopes (2H, 18O and 37Cl) in geothermal water, non-geothermal water (river water, cold groundwater) and seawater were used to identify the origin and evolution of geothermal water in coastal plain of Southwest of Guangdong. Two separate groups of geothermal water have been identified in study area. Group A, located in inland of study area, is characterized by Na+ and HCO3−. Group B, located in coastal area, is characterized by Na+ and Cl−. The relationships of components vs. Cl for different water samples clearly suggest the hydrochemical differences caused by mixing with seawater and water–rock interactions. It's evident that water–rock interactions under high temperature make a significant contribution to hydrochemistry of geothermal water for both Group A and Group B. Besides, seawater also plays an important role during geothermal water evolution for Group B. Mixing ratios of seawater with geothermal water for Group B are calculated by Cl and Br binary diagram, the estimated results show that about

Carlino S., Troiano A., Di Giuseppe M.G., Tramelli A., Troise C., Somma R., De Natale G.

The active volcanic area of Campi Flegrei represents one of the “hottest” sites of worldwide continental areas. Exploitation for geothermal energy of this volcano is of great interest, because temperatures>150 °C occur at very shallow depth (0.5–1 km). Since present time, the exploitation of geothermal energy in Italy, for electric uses, has been confined to the Larderello and Mt. Amiata districts (Tuscany). With the recent introduction of new Italian regulations, which favor and incentivize innovative pilot power plants (5 MWe), many geothermal projects have been applied to volcanic districts of Southern Italy, providing a new important input to the development of zero-emission geothermal power plants. In this framework, we analyzed the sustainability of geothermal exploitation in the high temperature geothermal field of Mofete (Campi Flegrei caldera) in Southern Italy. By a review of all the available data of drillings performed at Mofete, from 1979 to 1987 by AGIP and ENEL companies, and using a numerical simulation (TOUGH2 ® ), we evaluate the thermal and pressure perturbation of the reservoirs, and the possible induced seismicity, due to extraction and reinjection of fluids, at different depths and temperatures. The results are fundamental in planning a sustainable geothermal energy production in urbanized and high volcanic risk areas.

Sanjuan B., Millot R., Ásmundsson R., Brach M., Giroud N.

This work has made it possible to obtain two new Na/Li geothermometric relationships in addition to the three already known (Fouillac and Michard, 1981; Kharaka et al., 1982) and confirms that the Na/Li geothermometer, unlike the Na/K, Na/K/Ca, K/Mg and silica geothermometers, or the isotope δ 18 O (H 2 O–SO 4 ) geothermometer, also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water. One of the relationships concerns the fluids derived from seawater–basalt interaction processes existing in emerged rifts such as those of Iceland (Reykjanes, Svartsengi, and Seltjarnarnes geothermal fields) and Djibouti (Asal-Ghoubbet and Obock geothermal areas), or in numerous oceanic ridges and rises (Mid-Atlantic and Middle-Valley ridges, East Pacific rise, etc.). The best adapted Na/Li relationship for geothermal fluids discharged from emerged rifts between 0 and 365 °C is: T K = 920 / ⁢ log ⁡ N a / L i − 1.105 r 2 = 0.994 , n = 27 where Na and Li are the aqueous concentrations of these elements given in mol/L. The other Na/Li relationship was determined using dilute waters collected from wells located in different high-temperature (200–325 °C) volcanic geothermal areas of Iceland (Krafla, Námafjall, Nesjavellir and Hveragerdi). This relationship can be expressed as follows:T(K) = 2002/ [log(Na/Li) + 1.322] (r 2 = 0.967, n = 17). These two relationships give estimations of temperature with an uncertainty close to ± 20 °C. The second Na/Li relationship was also successfully applied to HT dilute geothermal waters from the East African Rift (Ethiopia, Kenya). Some case studies in the literature and thermodynamic considerations suggest that the Na/Li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite, K-feldspar, quartz and clay minerals such as kaolinite, illite (or muscovite) and Li-micas. Unlike the Na/Li ratios, no thermometric relationship using Li isotopes could be determined for this type of water. However, it was noticed that δ 7 Li values higher than 16‰ are always associated with low- to medium-temperature waters. • Two new Na/Li thermometric relationships are presented for geothermal exploration. • One concerns the fluids derived from seawater–basalt interaction processes. • The other can be applied on HT dilute geothermal waters in contact with basalts. • These relationships are controlled by chemical reactions involving illite and micas. • No thermometric relationship using Li isotopes could be determined.

Wang X., Wang G., Gan H., Zhang Y., Gao Z.

Tao L., Zhang Y., Yuan X., Chen Q., Yu J., Ma Y., Liu H., Tu C.

Han S., Nan D., Liu Z., Gesang N., Bianma C., Zhao H., Zheng Y., Xiao P.

Zuogong County is located in the southeast of Tibet, which is rich in hot spring geothermal resources, but its development and utilization degree are low, and the genetic mechanism of the geothermal system is not clear. Hydrogeochemical characteristics of geothermal water are of great significance in elucidating the genesis and evolution of geothermal systems, as well as the sustainable development and utilization of geothermal resources. The hydrogeochemical characteristics and genesis of the geothermal water in Zuogong County were investigated using hydrogeochemical analysis, a stable isotope (δD, δ18O) approach, and an inverse simulation model for water–rock reactions using the PHREEQC. The results indicated that the Zuogong geothermal system is a deep circulation heating type without a magmatic heat source. The chemical types present in the geothermal water from the Zuogong area are HCO3 and HCO3·SO4, and the main cations are Na+ and Ca2+. The groundwater is replenished by atmospheric precipitation and glacier meltwater. The salt content of geothermal water mainly comes from the interaction between water and surrounding rocks during the deep circulation process. The reservoir temperature of geothermal water in Zuogong is 120–176 °C before mixing with non-geothermal water and drops to 62–98 °C after mixing with 58 to 79% of non-geothermal water. According to the proposed conceptual model, geothermal water mainly produces water–rock interaction with aluminosilicate minerals in the deep formation, while in shallow areas it interacts mainly with sulfate minerals. These findings contribute to a better understanding of the geothermal system in Zuogong County, Tibet.

Bao Y., Pang Z., Li Y., Tian J., Luo J., Fan Y., Yang F., Qian T., Chen F., Sun C., Zhou Z.

Liu L., Wang G., Li Y., Wei Z., Lin W., Qin X., Zhang M., Qi S., Long X.

Liu J., Ren Z., Yu Q., Yan X., Qi K., Wang Z., Lan H., Jia M., Liu Y., Wu H.

Investigating hydrogeochemistry in geothermal fluids is a valuable approach to comprehending the intricate process of deep geothermal water circulation and uncovering the underlying mechanism behind the formation of geothermal systems. The article presents a thorough investigation of the hydrogeochemical and stable isotope characteristics of geothermal fields situated on both the southern and northern sides of the Xi'an depression. This study elucidates the process and progression of deep geothermal water, thereby offering theoretical backing for the exploitation of geothermal resources in the Weihe Basin. The data indicates the following points. The predominant chemical composition of geothermal water consists of SO4·HCO3–Na and SO4·HCO3·Cl–Na types. The ionic components are mainly impacted by the dissolution of Silicate and evaporite minerals, as well as the alternating adsorption of cations. The geothermal water is replenished by rainfall from the Qinling Mountains, with the recharge elevation varying from 677.94 m to 1,467.65 m. Various techniques are employed to determine the temperature and depth of the reservoir, which helps to understand the behavior of the deep thermal water. Thus, the study determines that the geothermal water in the Xi'an depression originates from laminar-controlled geothermal reservoirs, lateral flow recharge, and an unusual deep thermal structure.

Huang R., Zhang C., Jiang G., Zhang H.

AbstractEfficient exploration of geothermal resources is the basis of exploitation and utilization of geothermal resources. In recent years, Geographic Information System (GIS) has been increasingly used for the exploration owing to its power ability to integrate and analyze multiple sources of data related to the formation of geothermal resources, such as geology, geophysics, and geochemistry. Correctly understanding the control effect of evidence factors on geothermal resources is the premise and basis of whether the prediction results of evidence weight model are accurate. Traditionally, the conventional weight of evidence model assume that each evidence factor exerts a uniform controlling effect on the formation and distribution of geothermal resources. However, recent research indicates significant variations in the controlling ability of factors such as faults and granites, influenced by factors like activity levels and crystalline ages. Yet, studies addressing this differential control are lacking. To address this gap, we propose a series of weight of evidence models using abundant geological, geophysical, and geothermal data from the western Sichuan plateau, a high-temperature geothermal hotspot in China. This study aims to investigate the impact of varying controlling abilities of evidence factors on the evaluation model, with faults and granites as a case. Performance metrics include prediction rate, success rate index, receiver operating characteristic curve (ROC) and prediction rate of geothermal well. The findings of this research reveal that the weight of evidence model developed through the methodology outlined in this study exhibits superior performance compared to the conventional weight of evidence model. This superiority is evidenced by higher prediction rates, success indices, prediction rate of geothermal wells, and larger AUC values of ROC. Among these models, the weight of evidence model considering both fault and granite classification have the best performance in model evaluation indicators, with a prediction rate of 22.528 and a success index of 0.015408 in the very high potential area. The prediction rate and success index of the high potential area are 3.656 and 0.0025, respectively, and the prediction rate and success index of the middle potential area are 1.649 and 0.001128, respectively, and the AUC value is 0.808, indicating that the model has good accuracy. In terms of geothermal well prediction, the total prediction rate of geothermal favorable areas based on fault and granite classification evidence weight model is as high as 47.0526. Therefore, when constructing the weight of evidence model, the influence of the difference control of evidence factors on the formation of geothermal resources should be fully considered. These results underscore the effectiveness of the proposed methodology in enhancing the predictive accuracy and reliability of geothermal resource assessment in this study. Based on the prediction results of the weight of evidence model considering both fault and granite classification, four favorable geothermal areas with abundant surface heat display are identified in this paper, namely Kangding, Litang, Batang and Ganzi-Dege. In addition, the relatively weak surface heat display areas such as Jiulong, Daofu, Luhuo and Derong also show high geothermal potential. Some attention should be paid to geothermal exploration in the future.

Yan Y., Zhang Z., Zhou X., Wang G., He M., Tian J., Dong J., Li J., Bai Y., Zeng Z., Wang Y., Yao B., Xing G., Cui S., Shi Z.

The geochemistry of the fluids is spatially closely related to faults and earthquakes that have been detected in the northern Sichuan-Yunnan (SC-YN) block. However, the processes and factors that control hydrothermal fluid circulation on a regional scale remain challenging. The geochemical circulation of the hot springs integrated with geological and geophysical data revealed the processes and controlling factors of the relationship between fluid circulation and tectonic activity. Our results showed that the hydrochemical type of the hot springs at the intersection of the Yushu-Ganzi fault (YS-GZF), Xianshuihe fault (XSHF) and Yunongxi fault (YNXF), Jinshajiang fault (JSJF) and Batang fault (BTF) as well as Ganzi-Litang fault (GZ-LTF) is composed of Na-HCO3 and Na-HCO3·Cl. It demonstrated higher reservoir temperature and circulation depth (5 − 7 km). The helium isotope (0.22 − 2.36 Ra) of the gas indicated that the mantle helium composition was up to 5 − 30 %. These hot springs may undergo water–rock-gas exchange reactions with deep fluids. Considering geological and geophysical background as well as previous studies, the geochemical characteristics of the hot spring at the intersections of faults in the north of the SC-YN block are thought to be controlled by the deep-driven upwelling lower crustal flows, mixing with shallow hot spring water and the temporal and spatial heterogeneity of the high strain fields, slip rates, and permeability of the fault intersections. Whereas, the transport of fluids in hot springs of fault may increase pore fluid pressure, and reduce the effective stress at the intersection of the fault zones, which can promote fault sliding and provoke seismic events as well as fault movements. It could result in the enhancement of regional tectonic activity. This study provides an important basis for seismic prediction with hot spring hydrochemistry stations.

Luo Z., Yang M., Zhou X., Liu G., Liang J., Liu Z., Hua P., Ma J., Hu L., Sun X., Cui B., Wang Z., Chen Y.

The energy inside the Earth can not only be released outward through earthquakes and volcanoes but also can be used by humans in the form of geothermal energy. Is there a correlation between different forms of energy release? In this contribution, we perform detailed seismic and geothermal research in the Beijing area. The results show that the geothermal resources in Beijing belong to typical medium-low temperature geothermal resources of the sedimentary basin, and some areas are controlled by deep fault activities (e.g., Xiji geothermal well (No. 17)). The heat sources are upper mantle heat, radioactive heat in granite, and residual heat from magma cooling. The high overlap of earthquakes and geothermal field locations and the positive correlation between the injection water and earthquakes indicate that the exploitation and injection water will promote the release of the earth’s energy. The energy releases are partitioned into multiple microearthquakes, avoiding damaging earthquakes (ML ≥ 5) due to excessive energy accumulation. Therefore, the exploitation of geothermal resources may be one way to reduce destructive earthquakes. Furthermore, the use of geothermal resources can also reduce the burning of fossil energy, which is of great significance in dealing with global warming.

Liu H., Wu D., Wei W., Fang T., Cheng C., Cheng P., Gao X., Song Y., Huang J.

The Dabie Orogenic Belt is rich in geothermal resources, while relevant research is insufficient. To further find out the reserves of geothermal resources and their sources in the Dabie Orogenic Belt, reveal the mechanism of thermal circulations, and conduct quantitative assessments of the recharge sources, temperature of thermal reservoirs and depth of thermal circulations in the research area, on basis of thorough understanding of the geological conditions of the geothermal resources in the Dabie Orogenic Belt, four groups of geothermal water samples and six groups of underground cold water samples were collected in Xifei Geothermal Field for quantitative calculation, so as to test the hydrochemical characteristics and analyse the evolution process of geothermal water. The calculation results show that the geothermal water in Xifei Geothermal Field can be classified into SO4–Na type, and the main anions and cations in it derive from the dissolution of silicate and gypsum minerals. The data of δD and δ18O shows that the geothermal water is mainly recharged by the percolation of meteoric water. The temperature of geothermal reservoirs calculated with quartz and chalcedony geo-thermometers and by simulated multi-mineral equilibrium ranges from 85.05 to 116.31 °C.

Wang B., Zhou X., Li J., Zhang Y., Shen J., Zhong J., Bao Z.

Important fault zone processes can be discerned from the hydrothermal fluid circulation associated with active seismic sources. This study focused on the geochemical and isotopic features of the thermal waters along the Ganzi-Yushu fault (GZYSF) located in the eastern Tibetan Plateau. 22 spring samples were collected to characterize the chemical compositions and calculate the geothermal reservoir temperatures. The hydrothermal waters are mainly recharged by meteoric water based on the δ18O and δ2H values. Immature waters with a Na + -HCO3- or Ca2+(Mg2+)-HCO3- composition were recognized in the study area, combined with the poor correlation between K+, Na+, Mg2+, HCO3− and Cl− suggesting a shallow hydrothermal fluid source. The 87Sr/86Sr ratios and high concentrations of B (from 20.3 to 9445 μg/L) and Li (from 3.04 to 2380 μg/L) show a dissolution of silicate minerals containing calcium and magnesium during fluid circulation. The geothermometry in the silicon-enthalpy system suggested equilibrium temperatures up to 146 °C, whereas most are low temperatures. The shallow geothermal fields along GZYSF show that most large earthquakes (M>5) were distributed in the transitional zone between high-value area and low-value area of the shallow geothermal field, suggesting a potential genetic link between geothermal field structure and earthquake nucleation along active faults. In conclusion, we proposed a conceptual model to portray the fluid circulation process. Such work can provide insights into the potential geothermal resource in Tibetan Plateau, and also investigate the processes controlling the fluid chemistry and the correlation with occurrence of earthquakes in the region.

Yang H., Yuan X., Chen Y., Liu J., Zhan C., Lv G., Hu J., Sun M., Zhang Y.

The Yangbajing geothermal field, a renowned high-temperature geothermal resource in Tibet of southwestern China, has been utilized for power generation for several decades. To improve geothermal exploitation in the Yangbajing, genesis and mineral scaling have yet to be further revealed. In this study, hydrochemistry and D-O-Sr isotopy were employed for analyzing genesis and mineral scaling in the Yangbajing geothermal field. The geothermal waters were weakly alkaline and had a high TDS content (1400–2900 mg/L) with the Cl-Na, Cl·HCO3-Na, and HCO3·Cl-Na types. The dissolution of silicate minerals (sodium and potassium feldspars) and positive cation exchange controlled the hydrogeochemical process. The geothermal water was recharged from snow-melted water and meteoric water originating from the Nyainqentanglh Mountains and Tangshan Mountains. The geothermal waters possessed the highest reservoir temperature of 299 °C and the largest circulation depth of 2010 m according to various geothermometers. The geothermal waters can produce CaCO3 and SiO2 scaling during vertical and horizontal transport. These achievements can provide a scientific basis for the sustainable development and conservation of the high-temperature geothermal resources in Yangbajing and elsewhere.

Huang S., Rao S., Hu S., Zhang C., Lu J., Zhang Q., Gao T.

The western Sichuan in the eastern Qinghai-Tibet plateau has significant geothermal potential. Heat flow is one of the most important parameters in geothermic, its measurement can help obtain a better understanding of the regional lithospheric thermal structure. In this study, we reported 4 heat flow data based on the continuous steady-state temperature logging and rock thermal conductivity test, including two class-A and two class-D data. Furthermore, we calculated the lithospheric thermal structure by a 2D steady-state thermal conduction equation in COMSOL, analyzing the lateral differences of lithospheric thermal structure in the eastern Qinghai-Tibet plateau. Our study has determined that the heat flow values in the western Sichuan range from 94.7 to 116.2 mW/m2, with a corresponding geothermal gradient of 32.1–37.3 °C/km. Moreover, numerical simulation results indicate significant differences in lithospheric thermal structure between western Sichuan and Sichuan Basin, where the Longmen Shan fault zone is a crucial regional tectonic boundary and a significant temperature gradient zone. Specifically, the Moho temperature in the western Sichuan region ranges from 1000 to 1200 °C with an average temperature of 1060 °C, while the Sichuan Basin ranges from 600 to 700 °C. Besides, the thermal lithosphere thickness in the western Sichuan and the Sichuan Basin is 88–97 km and 105–110 km, respectively. These observations suggest that the western Sichuan region is characterized by high thermal anomaly and provide theoretical evidence for the geodynamic and geothermal development in this area. Finally, we suggest that the high thermal anomaly in western Sichuan is the result of a combination of factors, such as thickened crust, high radioactive heat production, and shear frictional heating from deep fault and so on.

Wang B., Qin X., Ren E., Feng N., Yang S., Li W., Li G., Jiang Z.

The Reshui area, located to the northeast of the Qinghai–Tibet Plateau, exhibits complex geological conditions, well-developed structures, and strong hydrothermal activities. The distribution of hot springs within this area is mainly controlled by faults. In this paper, five hot springs from the area were taken as the research object. We comprehensively studied the geochemical characteristics and genetic mechanism of the geothermal water by conducting a field investigation, hydrogeochemistry and environmental isotopic analysis (87Sr/86Sr, δ2H, δ18O, 3H). The surface temperature of the geothermal water ranges from 84 to 91 °C. The geothermal water in the area exhibits a pH value ranging between 8.26 and 8.45, with a total dissolved solids’ (TDS) concentration falling between 2924 and 3140 mg/L, indicating a weakly alkaline saline nature. It falls into the hydrochemical type CI-Na and contains a relatively high content of trace components such as Li, Sr, B, Br, etc., which are of certain developmental value. Ion ratio analysis and strontium isotope characteristics show that the dissolution of evaporite minerals and carbonate minerals serves as a hot spring for the main source of solutes. Hydrogen and oxygen stable isotope characteristics findings indicate that the geothermal water is primarily recharged via atmospheric precipitation. Moreover, the tritium isotopic data suggest that the geothermal water is a mixture of both recent water and ancient water. Moreover, the recharge elevation is estimated to be between 6151 and 6255 m. and the recharge area is located in the Kunlun Mountains around the study area. The mixing ratio of cold water, calculated using the silicon enthalpy equation, is approximately 65% to 70%. Based on the heat storage temperature calculated using the silicon enthalpy equation and the corrected quartz geothermal temperature scale, we infer that the heat storage temperature of geothermal water in the area ranges from 234.4 to 247.8 °C, with a circulation depth between 7385 and 7816 m. The research results are highly valuable in improving the research level concerning the genesis of high-temperature geothermal water in Reshui areas and provide essential theoretical support for the rational development and protection of geothermal resources in the area.

Wang L., Liu K., Zhang S., Zhang Y., Jia W., Yu T., Guo J.

The escalating issues of worldwide energy scarcity and environmental contamination have brought geothermal resources into the spotlight as a sustainable and eco-friendly energy alternative. The circum-Wugongshan geothermal belt has abundant geothermal resources at a medium-low temperature, offering significant potential for development and utilization. In this study, samples of geothermal groundwater, cold spring water, and surface water were collected from the western Wugongshan area. Hydrochemical and thermodynamic methods were used to estimate the reservoir temperature and analyze its mechanism of origin. The results of these analyses indicated that, in terms of hydrochemical characteristics, most geothermal groundwater samples were Na-HCO3 and Na-SO4, while cold spring and surface water samples were Na-HCO3 and Ca-HCO3, respectively. Quartz and multicomponent geothermometers provided the most reliable estimations of reservoir temperatures, ranging from 64.8°C to 93.4°C. The circulation depth of geothermal water was found to range from 1729.3 m to 2,292.5 m. A mixing model indicated that shallow cold water was blended at a rate of 62.1%–78.8%. The δD and δ18O values validated the conclusion that the geothermal water originates from atmospheric precipitation, with recharge elevations varying between 503.1 m and 1,375.6 m. Based on the above analysis, a conceptual model is proposed to illustrate the mechanism of geothermal groundwater genesis.